1. Field of the Invention
The present invention relates to a ladder-type piezoelectric filter and a method of manufacturing such a ladder-type piezoelectric filter. More particularly, the present invention relates to a structure of the ladder-type piezoelectric filter and a method for assembling components of the ladder-type piezoelectric filter into a case.
2. Description of the Related Art
In a conventional ladder-type piezoelectric filter, series resonators, parallel resonators, and terminal plates are stacked in the thickness direction of the filter and connected to one another in order to define a ladder circuit. The resonators and terminal plates are disposed in a box-shaped case which has an opening portion at one surface thereof. The opening portion is covered by an insulating paper, and sealing resin is provided on the insulating paper, to thereby seal the case (see Japanese Patent Application Laid-Open (Kokai) No. 3-255713).
During assembly of a conventional ladder-type filter, resonators and terminal plates are inserted one by one into the opening portion of a case so as to be arranged parallel to the inner wall of the case. Consequently, only a very small space is left for the last component to be inserted into the case, resulting in great difficulty in inserting the last component into the case.
Also, some of the terminal plates are designed to produce spring force in order to provide proper contact pressure. However, due to the contact pressure, when a terminal plate or a resonator is inserted into the case, the electrode of the resonator rubs against a projection on the terminal plate, thereby damaging the electrode and resulting in substantially decreased electrical reliability.
The thickness of each terminal plate, the thickness of each resonator, and the interior dimensions of the case are designed such that a constant pressure is applied onto the components which are disposed in the case. Therefore, each of the components must have a precise thickness. However, since the resonators are made of a piezoelectric ceramic material, considerable variations in thicknesses result. Also, the terminal plates have varying thicknesses due to formation of the projections disposed thereon. As a result, a constant contact pressure is not obtained, thereby significantly decreasing electrical reliability.
Moreover, in the conventional ladder-type filter, the lead portion of a terminal is not reliably and securely supported and the positions of the lead portions of a completed filter vary accordingly, resulting in problems in mounting the filter on a printed board or the like.